Low Voltage Differential Signaling (LVDS) interfaces are increasingly used for the large scale integration of electronic consumer products. An LVDS receiver comprising at least one differential difference amplifier (DDA) is used to receive and amplify an LVDS signal for use by other electronic circuits.
An exemplary version of a conventional LVDS receiver 110 is illustrated in the schematic diagram of FIG. 1. This LVDS receiver 110 comprises two DDA's 120a,b, one differential amplifier 130, and a multiplexer 140. A conventional embodiment of the individual DDA's 120a and 120b is illustrated in the schematic diagram of FIG. 2. This DDA 120 comprises a first pair of p-channel metal-oxide-semiconductor (PMOS) transistors 210a,b and a second pair of PMOS transistors 220a,b. The DDA 120 further comprises four n-channel metal-oxide-semiconductor (NMOS) transistors 230a-230d and four PMOS transistors 240a-240d, configured as shown in FIG. 2.
However, the conventional LVDS receiver 110 of FIG. 1 is susceptible to signal noise. Also, this LVDS receiver 110 is not capable of operating at low voltages, as sometimes required by mobile applications. Furthermore, this conventional LVDS receiver 110 can malfunction when a common mode voltage of an input voltage pair (PAD, PADN) is close to VSS, e.g., ground, while the common mode voltage of a reference voltage pair (Vref1, Vref2) is close to VDD/2, where VDD corresponds to a power supply voltage. In this situation, the drain voltages of the first transistor pair 210a,b, shown in FIG. 2, are pulled to approximately the level of the drain voltages of the second transistor pair 220a,b. The respective drain-to-source voltages of the first transistor pair 210a,b and the second transistor pair 220a,b cause the first transistor pair 210a,b to operate in a “triode” region, which is also referred to as a “resistive” region, while the second transistor pair 220a,b operates in a “pentode” region, which is also referred to as a “saturation” region. This difference in the respective operating regions of the two transistor pairs 210a,b, 220a,b results in an output voltage Vout at a node 250 having an amplitude that is not proportional to the difference between the differential of the input voltage pair (PAD, PADN) and the differential of the reference voltage pair (Vref1, Vref2).
Another version of an LVDS receiver is the conventional differential operational amplifier (op-amp) 310 illustrated in the schematic diagram of FIG. 3. This differential op-amp 310 comprises NMOS FETs 320a-320j, PMOS FETs 330a-330j, resistors 340a-340c, a capacitor 350, and an inverter 360, configured as shown in FIG. 3. The differential op-amp 310 may permit operation across a wider common mode range (CMR). However, this differential op-amp 310 is also susceptible to signal noise and generally incapable of operating at low voltages.
Thus, it is desirable to have a DDA adapted to operate effectively across a wider CMR. It is also desirable for the DDA to have increased tolerance to noise. It is still further desirable for the DDA to be able to operate at low voltages.